You'd still need to update and replace every system a packet would touch. Why just add another 8 or 16 bits and make it where we'd have to go through this entire painful process again? IPv6's design was "we never want to do this again".
An example of this "we never want to do this again" is only 1/8 of the v6 address space is currently marked usable for allocation. We have 7 more chances to change allocation methods without having to update or change any system.
This was a crazy episode